The study is concerned with the impact of mobile coronary care units (MCCU) on morbidity and mortality from acute myocardial infarction (AMI). Six hundred AMI cases treated in the prehospital phase by MCCU will be compared with 600 matched "control" AMI cases admitted to the same hospitals without receiving MCCU treatment. MCCU cases and "controls" will be investigated retrospectively to determine the course of events from onset of acute symptoms through admission to emergency room and hospital. Logs of the EMS Communication center will be used to identify cases treated by MCCU and transported to participating hospitals. Hospitals will maintain "registries" of all AMI cases admitted. The calls for help entering the EMS Communications Center will be transcribed for analysis. Clinical actions of the MCCU personnel will be reviewed and assessment of case severity made by a panel of cardiologists. In "control" cases the same procedures will be followed, except that there will be no record of prehospital care by the MCCU. All cases will be followed through hospitalization, obtaining clinical variables indicating case severity and complications, and six months after discharge, to ascertain patient outcome. The object of the study is to develop a methodology for evaluating and for comparing the effectiveness for such programs in different communities.